1. Field of Invention
The present invention pertains to the formation of aluminum containing films. More particularly, this invention pertains to chemical precursors for forming aluminum containing films.
2. Related Art
Aluminum oxide films have good electrical insulating properties and a high dielectric constant, which renders these films extremely advantageous as layer materials in integrated circuit designs. For example, aluminum oxide containing films are playing important roles in transistor and capacitor fabrication used in microprocessor and memory devices. Further, as integrated circuit device scaling dimensions continue to reduce into the nanometer scale, these films are likely to play a crucial role in forming dielectric films of future gate insulator layers in MOSFET transistors and capacitive structures of DRAM devices. In particular, recent advances in semiconductor technology have shown aluminum oxide containing films to be to be a superior material in certain critical applications such as the gate dielectric layer in MOSFET transistors and the capacitive storage media in DRAM memory devices.
In addition to having desirable electrical properties, Al2O3 films have good chemical inertness and mechanical strength properties. Thus, Al2O3 films are also used extensively in non-electronic circuit applications such as protective layers on hard disk heads, integrated circuit packaging, and anti-corrosion barriers for metal alloys.
Deposition of Al2O3 has traditionally been accomplished by means of chemical vapor deposition (CVD) or atomic layer deposition (ALD) using various aluminum precursors (e.g., alkyl aluminum and alkyl aluminum hydride compounds) in conjunction with an oxygen source. In thin film deposition for semiconductor fabrication, ALD is particularly desirable because of the need to deposit very thin, very uniform layers of, e.g., Al2O3 in continually shrinking semiconductor devices. One class of precursors commonly used in CVD includes the trialkylaluminum precursors, such as trimethyl aluminum (TMA). See, e.g., M. Gustin and R. G. Gordon, Journal of Electronic Materials, Volume 17, pages 509–517 (1988).
Aluminum isopropoxide has also been used as a precursor to deposit aluminum oxide. See, e.g., J. A. Aboaf, Journal of Electrochemical Society, Volume 114, pages 948–952 (1967). However this compound polymerizes easily and thus exists as a mixture of isomers, each with a different vapor pressure. Consequently, the vaporization of this precursor is unpredictable and difficult to control. In addition, aluminum 2-ethyhexanoate has been demonstrated as a precursor for Al2O3 deposition, but its low vapor pressure results in low deposition rates, which limits the usefulness of this compound in situations where high throughput is desired. See, e.g., T. Maruyama and T. Nakai, Applied Physics Letters, Volume 58, pp. 2079–2080 (1991).
One disadvantage associated with using alkyl aluminum compounds as precursors to form aluminum oxide films is that many are pyrophoric (i.e., igniting spontaneously in air) and hence present significant safety hazards in their use. There is a strong interest in industry today to avoid such risk if possible. A related disadvantage is the high reactivity of these compounds with oxygen that can result in the formation of powdered alumina in the vapor phase in a CVD chamber, particularly when excess oxygen is present.
The following sets forth a list of volatile Al compounds that can potentially be used for the various film deposition applications:
Phase atPyrophoric orCompoundTemperature/PressureNon-pyrophoricAlH(Me)2liquid at ~65° C./2.13 kPapyrophoricAl(Me)3liquid at 20° C./1.07 kPapyrophoricAl(Et)2ClliquidpyrophoricAl(Et)3liquid at 186° C./101.3 kPapyrophoricAl(Acac)3solid at 150° C./0.13 kPanon-pyrophoricAl(6Facac)3solid at 50° C./13.3 Panon-pyrophoricAl(Thd)3solid at 150° C./1.33 Panon-pyrophoricAl(i-OPr)3solid at 132° C./0.80 kPanon-pyrophoricAl(t-OBu)3156° C./0.27 kPaAl2(Et)3(s-OBu)3liquid at ~185° C./5.33 kPanon-pyrophoricwhere:Me = methyl;Et = ethyl;Opr = propoxide;Obu = butoxide;Acac = acetylacetonate (2,4 pentanedionate);Thd = tetramethylheptanedionate; and6Facac: hexafluoroacetylacetonate.
Several of the potential precursors listed above have sufficient volatility for effective use in an aluminum deposition process. However, many of these compounds are also pyrophoric and present substantial safety risks in their use, particularly in industrial environments. In addition to pyrophoricity, these compounds contain aluminum-carbon (Al—C) bonds that are undesirable in high dielectric insulating applications due to the potential of carbon incorporation of the film, which in turn leads to reduced resistivity. This has obvious disadvantages in gate dielectric and capacitor films as it leads to an increase in leakage current.
In addition, many of the remaining non-pyrophoric candidates in the above list contain aluminum-oxygen (Al—O) bonds. These bonds are quite strong (122 kcal/mol) compared to aluminum-nitrogen (AlN) bonds (71 kcal/mol), and thus it is quite difficult to substitute nitrogen for oxygen in a deposition process. To do so would require an additional process step in the overall film deposition (typically plasma nitridation). Increasing process steps decreases overall reliability, a factor that is critical for sustaining economic competitiveness in the semiconductor industry today.
Further use of higher energy processes, such as high temperatures or plasma, increases processing operating costs as well and can be detrimental to the quality of the critical gate dielectric layer of transistors in CMOS devices. Thus, it is desirable to utilize a precursor without a strong Al—O bond to form an aluminum oxide or other film, as this type of precursor provides the advantage of process tunability between oxide and nitride via process chemistry.
Aluminum dimethyl amide has been used to deposit aluminum nitride (AlN) films (see, e.g., J. Vac. Soc. Technol., 1996, 14, 306). In principle, it would appear that this compound might also yield Al2O3 in the presence of an oxidizer. However, this compound exists as a dimer and is solid at room temperature. Both of these attributes are not good characteristics for a vapor deposition process.